Enhanced thermal management solutions for RF power amplifiers
Synthetic diamond heat spreaders and GaN-on-Diamond wafers have emerged as a leading thermal-management technology for RF Power Amplifiers
GaN-based transistors and their related RF Power Amplifiers (PAs) have emerged as the leading solid-state technology to replace traveling wave tubes in radar, EW (Electronic warfare) systems, and satellite communications, and to replace GaAs transistors in cellular base stations. However, significant thermal limitations prevent GaN PAs from reaching their intrinsic performance capability. Metallized synthetic diamond heat spreaders have recently been used to address this thermal management challenge, particularly in cellular base station and military radar applications.
This article covers several important issues that advanced thermal solutions, particularly for RF power amplifiers, must address. Here, we are presenting new materials, such as CVD (chemical vapor deposition) diamond as a heat spreader to reduce overall package thermal resistance compared to today’s more commonly used materials for thermal management. Also, mounting aspects and some new developments regarding the thermal resistance at the bonding interfaces to diamond heat spreaders are discussed.
Diamond possesses an extraordinary set of properties including the highest known thermal conductivity, stiffness and hardness, combined with high optical transmission across a wide wavelength range, low expansion coefficient, and low density. These characteristics can make diamond a material of choice for thermal management to significantly reduce thermal resis- tance. CVD diamond is now readily commercially available in different grades with thermal conductivities ranging from 1000 to 2000 W/mK. Also very important is the fact that CVD diamond can be engineered to have fully isotropic characteristics, enabling enhanced heat spreading in all directions. Figure 1 shows a comparison of the thermal conductivity of CVD diamond with other materials traditionally used for heat spreading purposes.
|Figure 1. Comparison of thermal conductivity of CVD diamond and traditional heat spreading materials [1, 2].|
On-going development in the technologies to synthesize CVD diamond has enabled it to become readily available in volume at acceptable costs. Unmetallized CVD diamond heat spreaders are available today at a typical volume cost of $1/mm3. Prices vary dependant on the thermal-conductivity grade used. In some instances, system operation at elevated temperatures can reduce both the initial cost of the cooling sub-system and the on-going operating cost as well. When applied with appropriate die-attach methods, diamond heat spreaders provide reliable solutions for semiconductor packages with significant thermal management challenges .
Application notes for the use of CVD Diamond
To obtain the most effective use of the extreme properties of CVD Diamond in overall system design, package integration issues need to be carefully considered. Failure to address any one of these issues will result in a sub-optimal thermal solution. Here are the most important points to be considered:
- Surface preparation
- Mounting techniques
- Diamond thickness
- Functional considerations
- Metallizations and thermal barrier resistance
Surface preparation: The surfaces of die-level devices have to be machined in a suitable fashion to allow good heat transfer. Surface flatness for heat spreaders should typically be less than 1 micron/mm and the roughness better than Ra < 50 nm, which can be achieved by polishing techniques. Any deficiency in flatness must be compensated for by the mounting techniques which will cause higher thermal resistance.
Mounting techniques: Whereas in some advanced device applications, such as high-power laser diodes, atomic-force bonding techniques are being considered, most applications currently employ soldering techniques for die attachment to the heat spreader. Again, solder layers should be kept to minimum thickness, particularly for the primary TIM1 (thermal interface material (TIM) between die and heat spreader), to minimize thermal resistance. An important factor in applying solder joints is the expansion mismatch between the CVD diamond and the semiconductor material, as it can significantly influence performance and lifetime. GaAs (Gallium Arsenide) devices up to an edge length of 2.5 mm can be hard soldered to CVD diamond without CTE-mismatch problems. (Note that the CTE for CVD Diamond is 1.0 ppm/K at 300K). For edge lengths greater than 2.5 mm, using a soft solder can avoid excessive stresses in the device. TABLE 1 shows a wide range of solder materials commercially available to address various needs for soldering processes.
|Table 1. Summary of soldering materials.|
Diamond thickness: The thickness of the CVD diamond is important. For devices with small hot spots, such as RF amplifiers or laser diodes, a thickness of 250 to 400 microns is sufficient. Diamond’s isotropic characteristics effectively spread the heat to reduce maximum operation temperature at constant power output. However, applications with larger heat spots on the order of 1 to 10 mm in diameter require thicker diamond for better results. An example is disk lasers that can have an optical output power of several kW and a power density of about 2kW/cm2; a diamond thickness of several mm has proven to be beneficial to disk laser operation .
Functional considerations: There are also functional requirements that may be important. One is the electrical conductivity of the heat spreader. For devices such as laser diodes, it is easiest to run the drive current through the device and use the heat spreader for the ground contact. For other devices, the heat spreader is required to be insulating. As CVD diamond is an intrinsic insulator, this insulation can be maintained by keeping the side faces free of metallization. This could be required for RF amplifiers and transistors, especially at higher frequencies (f > 2 GHz).
Thermal simulation helps optimize the heat spreader configuration to find the best solution based on power output needs, material thickness, metallization scheme, heat source geometry and package configuration. For design optimization, it is important that the thermal simulation model includes the complete junction-to-case system, including the device details, all interfaces, materials and the subsequent heat sinking solution.
Metallizations and thermal barrier resistance
Metallizations are an essential component to the application of CVD diamond in RF Amplifier and similar applications. Typically, for reasons of adhesion, mechanical and thermal robustness, three-layer metallization schemes are used. An example of such a three-layer metallization scheme fundamentally comprises: a) a carbide forming metal layer which forms a carbide bonding to the diamond component; b) a diffusion barrier metal layer disposed over the carbide forming metal layer; and c) a surface metal bonding layer disposed over the diffusion barrier metal which provides both a protective layer and a wettable surface layer onto which a metal solder or metal braze can be applied to bond the diamond heat spreader to die and other device components. A particular example of such a three-layer metalli- zation scheme is Ti / Pt / Au.
High-quality, sputter-deposited, thin-film metallizations are strongly recommended for advanced thermal solutions. As thermal contact resistance between the device
and the heat spreader must be minimized, any additional metal interface being added to the system must be avoided. Sputtered layers, especially of titanium, can form a very effective chemical bond with CVD diamond to ensure long-term stability even at elevated temperatures. To separate the required gold attach layer from the titanium adhesion layer, a platinum or titanium/tungsten (TiW) barrier layer is recommended. The Ti/Pt/Au scheme is very commonly used in high-end devices and has excellent characteristics with regards to stability and endurance, even over extended lifetime periods under changing thermal loads. However, this scheme also has a drawback, as the thermal conductivities of the titanium and platinum are relatively low (Tc=22 W/ mK and Tc=70 W/mK respectively). In the search for improved materials to be applied, the use of chromium has been identified as a viable alter- native. Chromium forms a carbide with diamond and is also readily used as a barrier layer, enabling it to perform both functions at a relatively high thermal conductivity of Tc=93.9 W/mk. To test the thermal effectiveness of chromium, samples were prepared at the CDTR (Centre for Device Thermog- raphy and Reliability) at Bristol University comparing a standard Ti/Pt/Au (100/120/500nm) metallization with this novel Cr/Au (100/500nm) configuration. The measurements of the thermal conductivity revealed that the thermal conduc- tivity of the Cr/Au metallization is about 4 times higher as compared to the Ti/Pt/Au. Results are shown in Figure 2.
|Figure 2. Comparison of thermal conductivity of different metallization schemes|
To demonstrate the impact of this Cr adhesion/ barrier layer advantage versus Ti/Pt/Au, high power GaN on SiC HEMT (High Electron Mobility Transistor) devices were mounted to a CVD Diamond heat spreader. A cap layer of AuSn with a thickness of 25 microns was chosen. To ensure comparable results for all samples prepared, these samples were placed on a temperature stable platform also made from high thermally conductive diamond material. Results are shown in FIGURE 3: In the left diagram, the base temperature is plotted for increasing power output from the device. As can be seen, the temperature for the Cr/Au configuration is significantly lower, at 9W device power output by about 10 degrees C. On the right hand side, the graph shows the temperature as measured on the transistor channel directly.
|Figure 3. Temperatures as a function of power for different metallization schemes and solder thickness.|
In this case, the lower thermal resistivity of the Cr-based metallization layer decreases the channel temperature by more than 20 degrees C at 9W power output.
This significant temperature reduction will result in as much as a 4 times longer lifetime of the device. Alternatively, such devices could be packaged in smaller footprints, at higher power densities, to make use of this increased effectiveness in heat spreading.
Outlook, future developments
One important finding from the above example is the need to modify device architecture for improved thermal management. The main temperature rise is within the device itself. Here, a thinning of the substrate, to bring it closer to the diamond heat spreader, would further enhance the thermal design. Also, mounting such devices with the active layers facing the diamond would provide even further benefit. An example would be the mounting laser diodes p-face down with the quantum well structures soldered directly against the heat spreader. Another way to bring the device gate junction closer to the diamond is the use of a different substrate altogether. This has been demonstrated by using GaN (Gallium Nitride) on diamond wafers, which remove both the Si substrate and transition layers, replacing them instead with CVD diamond . The result brings the diamond material within 1 micron of the heat generating gate junctions. Initial users of GaN-on-diamond wafers for RF HEMT devices have demon- strated as much as 3 times the power density when compared to equivalent GaN/SiC (Silicon Carbide) devices, today’s leading technology for advanced power devices. 
As can be seen, significant thermal-management improvements to electronic systems can be realized by using advanced materials such as CVD diamond. The integration can be relatively straightforward as the diamond heat spreader can be a direct replacement to AlN (Aluminium nitride), BeO (Berillium oxide) or other advanced ceramics. Attention to detail at the interfaces, both in terms of the choice of metals and its thickness, is important to keep overall thermal resistance low and thereby optimizing the effectiveness of the diamond.
As CVD diamond becomes more attractive as a heat spreader through improved synthesis technology, advanced processing and on-going cost reduction efforts, its use in high power density applications has been increasing. It is expected that this trend will be continued in the years to come in line with the ever increasing need for smaller and more powerful electronic devices and systems.
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